Electrophotosensitive copy-sheet



United States Patent Office 3,010,884 Patented Nov. 28, 1961 3,010,884 ELECTROPHOTOSENSITIVE COPY-SHEET Edgar G. Johnson, St. Paul, Minn., and Byron W. Neher,

Hudson, Wis., assignors to Minnesota Mining and Manufacturing Company, St. Paul, Minn., a corporation of Delaware No Drawing. Filed Oct. 28, 1957, Ser. No. 692,529

4 Claims. (Cl. 204-18) This invention relates to strongly photoconductive lightsensitive sheet materials. While not restricted thereto, it has particular reference to copy-sheets for making visible reproductions of light-images impressed thereon, by methods involving continuous electrolysis of an electrolytic developer solution at surface areas exposed to the lightimage.

Photoconductive coatings and sheet materials have previously been suggested for various purposes, including the reproducing of light-images.

In the xerographic process, as described in Carlson Patent No. 2,297,691, a metal plate is provided with a very thin surface coating of photoconductive insulator material, e.g., sulfur or anthracene; the coating is electrostatically charged at a high voltage; a light-image is impressed on the charged surface, permitting corresponding areas to become discharged; and the remaining charge pattern is rendered visible by application of electrostatically charged powder. The powder pattern is subsequently transferred to another surface and fixed by fusion or otherwise, whereupon the coated plate may be re-used.

The more recently developed Electrofax process, described by C. LYoung and H. G. Greig at pages 469- 484 of the RCA Review for December 1954, uses an ordinary paper base coated with photoconductive zinc oxide in a resinous binder. The sheet is electrostatically charged, exposed to a light-image, and the remaining electrostatic latent image developed with a charged powder as in the xerographic process; but the image is permanently retained on the'coated paper, the powder being fused in place.

These prior art procedures require that the sensitized surface be capable of retaining a high voltage electrostatic charge under dark conditions, and of permitting such charge to be dissipated when the surface is exposed to light. Typically, the coating will have a resistivity in the dark of at least about 10 ohms/cm. and preferably about 10 ohms/cm, and will be sufiiciently photoconductive under illumination to permit a previously im pressed high voltage charge to be dissipated.

In copending application Serial No. 575,070, filed March 30, 1956, and of which the present application is a continuation-in-part, there is described a further method of developing permanent visible images on suitable photoconductive copy-papers exposed to light-images. The method involves the rapid continuous electrolysis of an electrolytic developer solution, and particularly the electrodeposition of a metallic or other visibly distinct coating, at the exposed photosensitive surface. No preliminary charging of the copy-sheet is required, and the copy produced needs no further heating or other processing to render the image permanent. However, the successful application of the electrolytic method has been found to require, among other things, that the sensitive sheet be strongly and continuously photoconductive rather than merely capable of holding and dissipating an electrostatic charge. The present invention is therefore directed to the preparation of such strongly photoconductive copypapers, with which electrolytic image-development may effectively be accomplished.

As an example of an important field of utility, this invention has made possible the direct copying of microfilm reproductions of printed pages of books or the like in the same dimensions as the original page and within a time of not more than about five or ten seconds from initial inspection of the light-image to delivery of the completed print. With microfilm in conventional roll form, prints of desired frames are produced at substantially no increase in the time required for merely scanning and see lecting the frames. Apparatus for effecting such operations is described and claimed in copending application Serial No. 686,237, filed September 25, 1957.

The intensity of illumination, time of exposure and development, and electrical potential which may safely and efficiently be employed in the operation of copying apparatus as just described are severely limited. For practical commercial operation it has been found desirable, for example, to employ conventional projection lamp sources of illumination, to restrict the time of exposure and development to not more than a few seconds, and to restrict the applied potential to not more than about 50-100 volts. The copy-sheet should be flexible for easy handling, yet the photosensitive material must remain a permanent component thereof.

As a specific example, permanent enlarged copies of microfilm originals are now successfully rapidly produced on the copy-sheet material of this invention supplied in roll form. A full letter-size light-image is projected to the copy-sheet from a 35-min. microfilm transparency of average optical density by means of a SOO-watt incandescent-filament projection lamp. After exposure for not longer than about five seconds, the exposed sheet is developed within a total further time not exceeding about five seconds, at a potential in the neighborhood of 20 volts D.C., to provide clear and distinct image areas of high contrast and excellent detail, requiring no fixing, washing or other additional processing. Under such conditions, sheet materials prepared in accordance with prior art teachings for use in the electrostatic methods of copying have been found to be completely inoperative.

In one instance, a suspension of photoconductive zinc oxide powder in a solution of a hard and somewhat brittle resinous binder, obtained from a commercial source and represented to be a coating composition as employed in the manufacture of Electro-Fax electrostatic copypaper, was applied as a thin uniform coating on bond paper and dried to remove volatile solvent. The coating penetrated the paper sufficiently to obtain effective bonding so that the sheet could be handled without significant loss of photoconductive material. The sheet was held under dark conditions and charged electrostatically from a probe maintained at 6000 volt negative potential and passed slowly above the sheet at a distance of about onehalf inch. The charged sheet was then exposed to a light-image, and the resulting electrostatic latent image was developed with a powdered dyed resin. A clear visible reproduction was obtained. On the other hand, the coated sheet, when exposed to a light-image and subjected to electrolytic developing conditions substantially as above defined, could not be made to yield a visible image.

The following example illustrates a specific formulation and procedure for the preparation of stronglyphotoconductive copy sheets useful in the copying of light images by the novel electrolytic methods herein described.

EXAMPLE 1 slurry or suspension is thick and viscous but flows readily and can be spread with a coating knife to form a smooth uniform coating.

The suspension is coated on the clean metal surface of a laminate of thin paper and thin aluminum foil, and the solvent removed by evaporation, to provide a smooth uniform dried coating about 0.8 mil thick. The resulting sheet is flexible and the coating remains firmly bonded to the metal during handling or rolling of the sheet.

Reproduction of impressed light-images may be produced on the surface of the copy-sheet of this example, prepared as just described, by either of the electrostatic or electrolytic methods previously identified. In the electrolytic method, a typical procedure is as follows:

The sheet, previously held under dark conditions for at least about one-half hour, is exposed to a light-image for about five seconds as previously noted. The negative pole of a 20 volt D.C. source of potential is then connected to the metal foil of the sheet, as by means of edge clamps, the positive pole being connected to a narrow strip of fine-grained cellulosic sponge partly saturated with an electrolytic developer solution of 3 parts by weight of cadmium nitrate tetrahydrate, 0.5 part each of tartar emetic and silver nitrate, and 100 parts of water. The sheet is drawn past the sponge at a constant rate such that each point of the surface remains in contact with the sponge for about 0.4 second. A dark deposit is formed at the light-struck areas, while the unlighted areas remain white. The sheet remains substantially dry. The dark image areas are effectively permanent.

The suspension of Example 1 was similarly coated on cellophane, which, because of a substantial content of humectant plasticizer, is found to be electrically conductive. The sheet produced excellent prints when charged, exposed, and developed by the electrostatic method. Under electrolytic development conditions, no visible copy could be'obtained on such sheet material.

The cellophane was moistened with water and the thin active coating removed as a continuous film. The film was contacted on the initially exposed face surface with the electrolytic developer solution and on the initially concealed back surface with another aqueous electrolyte solution to provide a contact medium, and an appropriate potential was impressed across the two solutions while a light-image was directed at the film surface. No visible deposit could be obtained under these conditions, whereas the coated foil and paper laminate of Example 2, tested under the same conditions, was fully effective.

The face surface of the free film was firmly pressed against a clean metal foil and again tested. Substantially no visible deposit was obtained by electrolytic development. On the contrary, fully eifective images were obtained with this structure when charged, exposed and developed in accordance with the electrostatic method.

The film was next metallized, by aluminum vapor deposition in vacuum, and again tested. When the metal was applied to the face surface of the film, no image could be produced on the back surface by electrolytic methods. However, a useful image could be produced by electrolytic development at the face side of the film which had been metallized on the back surface. It was observed that the back surface of the film removed from the cellophane was smooth, whereas the face surface was relatively rough and uneven when viewed under a microscope.

The paper-foil laminate used as the substrate or carrier in Example 1 was coated on the clean metal surface with the commercially available Electro-Fax coating composition previously described. The brittleness of the coating resulted in flaking and loss of active material when the sheet was flexed or shaken. Effective copies could be produced on the carefully handled sheet by the electrostatic method, but no visible image was obtained by electrolytic development techniques as hereinabove described.

As above noted, the copy-sheet of Example 1 was fully effective under commercially acceptable electrolytic deveioping procedures. It was observed, however, that light rubbing of the sensitve surface with a cloth moistened with a solvent for the polymeric binder rendered the surface completely inoperative toward such procedures. Similarly, coating the surface with a further very thin layer of a solution of the polymeric binder destroyed the effectiveness of the copy-sheet for electrolytic imagedevelopment. In both instances the sheet produced fully acceptable copies when processed in accordance with the electrostatic method.

Contamination of the aluminum surface is also found to destroy the effectiveness of the completed copy-sheet for electrolytic methods While still permitting successful electrostatic development. For example, a thin coating of polymer applied as a wash coat to the clean aluminum prior to application of the polymer-pigment mixture has completely prevented electrolytic development on the resulting sheet. Oily or greasy surface deposits on the foil are also detrimental. Surprisingly, attempts to clean the surface with alkali silicate solutions result in sheets which are even less susceptible to electrolytic evelopment. It has been found, however, that brief contact of a soiled aluminum foil surface with strong aqueous alkali, e.g. 50% potassium hydroxide solution, followed by thorough rinsing and drying, provides a fully effective surface. The same result may be obtained by vapor deposition of aluminum on the base surface under vacuum, preferably after preheating in the vacuum. Vapor deposition of aluminum on other substrates, e.g. on cellulose acetate film, has likewise produced an effective base material which when coated with the zinc oxide-polymer mixture of Example 1 has been found useful in electrolytic electrocopying procedures as hereinabove described.

The styrene-butadiene polymer employed (Pliolite 8-7) is soluble in toluol but insoluble in acetone. The addition of a further 650-700 grams of acetone to the approximately 1500 gram quantity of slurry as prepared in Example 1 results in the coagulation and precipitation of the binder and pigment. The much smaller amount of acetone present in the suspension as coated is insufficient to cause precipitation of any portion of the nonvolatile components, but nonetheless appears to have a desirable $1 set on the mixture both in respect to increasing the rate of drying of the coating and also in providing significant improvement in the ability of the copy sheet to undergo electrolytic image-development.

The utility of a photoconductive copy-sheet for electrolytic image-development may be forecast to a considerable degree by measuring the photoconductive value of the sheet. A small section of the sheet material is insulated at back and edge areas with a nonconductive waterproof covering, e.g. of plastic adhesive tape, an electrical connection to the conductive metallic substrate being provided. The sample is suspended in a transparent glass cell containing 200 ml. of a solution of tenthmolar ammonium sulfate and facing an open frame electrode serving as the anode. Current flow per unit area through the measured thickness of the coating under an applied voltage is measured both with the sample under equilibrium dark conditions and when illuminated. A potential of 10 volts is convenient but not critical. Values at several thicknesses of coatings may be determined and the value at a standard thickness obtained by interpolation. A coating thickness of 0.8 mil as thus determined is convenient. Illumination is provided from a 500 watt incandescent-filament lamp, i.e. at an intensity of about 1300 foot-candles. The photoconductivity value may be calculated from the values thus obtained and is conveniently reported as mho/cm. Development has been carried out electrolytically with copy sheets having a photoconductivity value, as thus determined, as low as about 1O' mho/cm. However for effective commercial operations as hereinbefore described, copy sheetshaving photoconductivity values not less than about 10' mho/cm. are much superior and are preferred, and still higher values have also been attained. In all cases, the conductivity values under dark conditions must not be higher than about one-twentieth of the photoconductivity value for best results in terms of electrolytic image-development.

Variations in the ratio of photoconduotivepowder and resinous binder material are found to produce significant variations in the effectiveness ofthe copy-sheet in 'elec trolytic image-development and analogously in the photoconductivity value as above defined. The following tabula-tion illustrates the results, in terms of photoconductivity values, obtained with a series of copy-sheets prepared generally in accordance with the teachings of Example 1 but employing different pigment-binder ratios. In all cases the amount of volatile solvent was controlled to pro vide a viscous spreadable suspension, and the suspension was ball-milled until smooth and free of lumps on coating. The suspension was coated on the clean metal surface of the metal-foil laminate as employed in Example 1. Typical values of the apparent density of the coating and of the reflectance value are included.

Properties of oxide-binder coatings EXAMPLE 2 A clean-surfaced aluminum foil and paper laminate as used in Example 1 was coated with a thin layer of a smooth suspension of 80 grams of USP-12 high conductivity zinc oxide in a solution, in 80 grams toluene, of 40 grams of DC 803 silicone solution (a 50% solution in xylol of alkyl aryl silicone resin capable of curing in one hour at 480 F. to a hard and somewhat brittle polymer). The suspension was milled in a one-pint ball mill with one-half inch porcelain balls for about 4 hours until smooth, and was coated at a thickness of 4-5 mils. After air drying the coating was about .8 mil thick. During the first several days the coating remained flexible and the sheet was highly effective as a copy-sheet for electrolytic image-development. Subsequently the coating became somewhat brittle for effective retention on a flexible copy-sheet.

The copolymer of styrene and butadiene employed in Example 1 as a binder for the light-sensitive zinc oxide is a water-resistant, flexible, adherent, film-forming poly mer of highly satisfactory properties. It is light in color, and does not interfere with the light-sensitivity of the pigment. It is readily soluble in low cost solvents, yet the solvent may be removed without difficulty by forced drying. The polymer is relatively inexpensive and readily available. More particularly, the polymer does not appear to wet the zinc oxide powder, at least to the extent necessary to form a continuous film over the particles. Other binders meeting most or all of theserequirements include polystyrene, chlorinated rubber,'-rubber hydrochloride, polyvinylidene chloride, nitrocellulose, polyvinyl butyral. On the other hand, polymers which are dissolved or softened by water, or which are dark in color, or insoluble in commercial solvents, or reactive with the pigment, or which readily wet the pigment particles, are

6 found to be ineffective. As typical examples, polyvinyl alcohol, polyacrylic acid, shellac, and sodium carboxymethyl cellulose are not acceptable as binders for the light-sensitive sheet materials of this invention.

The particular Zinc oxide powder specified in Examples 1 and 2 is a pigment grade oxide of high purity and relatively large particle size, and is a preferred highly photoconductive powder for the purposes of this invention. As an indication of the photoconductivity, tests have been made of the powder in compressed slab form in the absence of polymeric binder. In making the test, a weighed 200 milligrams of the powder is uniformly distributed within an open channel 3 cm. long and 0.63 cm. wide formed between two parallel aluminum strips mounted on a fiat Lucite plate. The powder was compressed with a close-fitting flat ram under a pressure of 214 lbs/sq. in., forming a compacted slab about 0.05 cm. thick connecting the aluminum strips. The block was maintained under anhydrous conditions over calcium sulfate and at normal room temperature. Current flow through the compacted slab between the aluminum strips was determined at an impressed potential of volts D.C. although results were substantially the same when the test was run at 10 volts. The sample was first placed under equilibrium dark conditions. It was then exposed to light from a 500 watt tungsten filament projection bulb operated at a color temperature of 3100 K., the light passing through a glass-walled water-filled filter cell and providing an illumination of approximately 0.019 watt/sq. cm. of radiant energy, about 0.005 watt being in the visible region between about 0.38 and about 0.70 micron wavelength. Finally, the sample was again returned to dark conditions and testing was continued until the rate of current flow had diminished to one-half the maximum obtained during illumination of the sample. From the measured current flow and dimensions of the test sample, the apparent conductivity was calculated to be as follows:

Apparent conductivity of compressed zinc oxide Equilibrium dark conditions 3.2X10- .mho /cm. Illuminated 5 seconds; 5.0 10" mho/cm. Half life 1 second.

The term apparent conductivity is used since the effect of the illumination is necessarily confined to the surface layers and does not extend uniformly throughout the thickness of the sample.

Other zinc oxides and other photoconductive powders having equivalent apparentphotoconductivity values in compressed slab form are also found to provide adequate photoconductivity values in coated film form and to produce copy-sheets susceptible of electrostatic imagedevelopment when suitable precautions are taken in selection and treatment of other components in accordance with disclosure herein provided. As one example, 2.35 parts by weight of a sample of high photoconductivity cadmium sulfide powder, having a compressed slab photoconductivity value in the test described above of 4X 10- mho/cm. and a dark conductivity of 4 10- mho/cm., was combined with one part of the polymeric binder of Example 1 and coated on clean metal foil to provide a useful copy-sheet having a dark conductivity value of 4.4 '10 mho/cm. and a photoconductivity value of 6x10 mho/cm. The sheet provided useful copy by electrolytic image-development, but required somewhat higher light-image intensity, or time of exposure, or impressed voltage than is desirable for many commercial operations, and in addition required that exposure and development beaccomplished simultaneously.

In the foregoing formulations the amounts of photoconductive powder and polymeric binder have been expressed for convenience in terms of weight proportions. A more accurate method of expression these relationships is in terms of volume. It is found that coatings in which the volume of the photoconductive particles come within the range of about 30-55% of the total volume of particles and binder provide the best results in terms of maximum photoconduetivity values and, more import-antly, in terms of high quality performance in electrolytic image-development. With significantly lesser amounts of the powder component in the sensitive coating, effective image-development by electrolytic means cannot be attained regardless of the compressed-slab photoconductivity value of the powder. With significantly greater amounts of the powder component, the electro lytic image-developing procedure is found to cause the formation of numerous dark spots on areas over the sensitive surface. Significantly, these wider ranges of proportions oifer no difiiculties in the electrostatic imagedeveloping procedures.

In the foregoing examples the strongly photoconductive coatings have been applied to clean aluminum surfaces; and such substrates are preferred both for reasons of availability and economy as well as providing desirable reflectivity and other properties. Other metallic substrates, such as silver and copper, may be substituted where suitable precautions are taken to provide a clean conductive surface. For many purposes the substrate need not be flexible. Thus, thick metal plates may be coated with the sensitive composition and images formed thereon by electrolytic development procedures as an aid in subsequent machining of the plate, in making meter dials or other printed metal articles, or for other purposes. In all cases it is necessary that a clean, electronically conductive carrier or substrate surface be provided, as distinguished, for example, from an ionically conductive material; and that the binder component permit the attainment of conductive contact between particle and substrate as well as between adjacent particles, while still holding the particulate layer to the conductive supporting surface; and further that the outer surface layer of particle be exposed for eventual direct contact with the electrolytic developer solution. The application of these requirements, taken together with the required high degree of photoconduetivity and relatively low dark conductivity of the photoconductive powder material as hereinbefore defined, has now been shown to provide for a copy-sheet material capable of rapidly yielding effective copies of light-images by novel electrolytic image-developing procedures not applicable to previously available photoconductive copy-sheet materials and having particular applicability to the rapid commercial preparation of enlarged permanent copies of microfilm originals.

What is claimed is as follows:

1. A flexible photoconductive copy sheet having a thin, uniform photoconductive layer comprising zinc oxide particles firmly held to a smooth continuous flexible aluminum layer by a substantially water-insoluble insulative organic resinous binder having a low degree of wettability toward said zinc oxide particles in such a manner that there is a lowermost stratum of zinc oxide particles directly contacting said aluminum layer and substantially all zinc oxide particles in said photoconductive layer are in direct contact with adjacent zinc oxide particles, said aluminum layer overlying and being attached to an electrical insulating, flexible backing sheet, the amount of said zinc oxide particles in said photoconductive layer being such that an electronically conductive path is created between the outer surface of said photoconductive layer and said aluminum layer upon irradiation of said photoconductive layer, and said aluminum layer being of such cleanness and said Zinc oxide particles being of such photoconduetivity that the combined aluminum and photoconductive layers have a conductivity of at least about 10- mho/cm. on exposure to light and a conductivity in the dark not greater than about one-twentieth of the conductivity on exposure to light.

2. A flexible, photoconductive copy sheet having a thin, uniform substantially white photoconductive layer comprising zinc oxide particles firmly held to a thin flexible aluminum foil layer by a copolymer of styrene and butadiene as a binder in such a manner that there is a lowermost stratum of zinc oxide particels directly contacting said aluminum foil layer and substantially all zinc oxide particles in said photoconductive layer are in direct contact with adjacent zinc oxide particles, said aluminum foil layer overlying and being attached to an electrical insulating, flexible paper backing sheet, the amount of said zinc oxide particles in said photoconductive layer being such that an electronically conductive path is created between the outer surface of said photoconductive layer and said aluminum foil layer upon irradiation of said photoconductive layer, and said aluminum layer being of such cleanness and said zinc oxide particles being of such photoconduetivity that the combined aluminum and photoconductive layers have a conductivity of at least about 10* mho/cm. on exposure to light and a conductivity in the dark not greater than about onetwentieth of the conductivity on exposure to light.

3. A flexible photoconductive copy sheet having a thin, uniform photoconductive layer comprising zinc oxide particles firmly held to a smooth continuous flexible metal layer by a substantially water-insoluble insulative organic resinous binder having a low degree of wettability toward said zinc oxide particles in such a manner that there is a lowermost stratum of zinc oxide particles directly contacting said metal layer and substantially all zinc oxide particles in said photoconductive layer are in direct contact with adjacent zinc oxide particles, said metal layer overlying and being attached to an electrical insulating, flexible backing sheet, the amount of said zinc oxide particles in said photoconductive layer being such that an electronically conductive path is created between the outer surface of said photoconductive layer and said metal layer upon irradiation of said photoconductive layer, and said metal layer being of such cleanness and said zinc oxide particles being of such photoconductivity that the combined metal and photoconductive layers have a conductivity of at least about 10" mho/cm. on exposure to light and a conductivity in the dark of at least 10- mho/cm. but not greater than 1about one-twentieth of the conductivity on exposure to ight.

4. A flexible photoconductive copy sheet having a thin, uniform substantially white photoconductive layer com prising zinc oxide particles firmly held to a smooth continuous flexible metal layer by a substantially water-insoluble insulative organic resinous binder having a low degree of wettability toward said Zinc oxide particles in such a manner that there is a lowermost stratum of zinc oxide particles directly contacting said metal layer and substantially all zinc oxide particles in said photoconductive layer are in direct contact with adjacent zinc oxide particles, said metal layer overlying and being attached to an electrical insulating, flexible paper backing sheet, the amount of said zinc oxide particles in said photoconductive layer being such that an electronically conductive path is created between the outer surface of said photoconductive layer and said metal layer upon irradiation of said photoconductive layer, and said metal layer being of such cleanness and said Zinc oxide particles being of such photoconduetivity that the combined metal and photoconductive layers have a conductivity of at least about 10* mho/cm. on exposure to light and a conductivity in the dark not greater than about one-twentieth of the conductivity on exposure to light.

References Cited in the file of this patent UNITED STATES PATENTS 2,297,691 Carlson Oct. 6, |1942 2,554,017 Dalton May 22, 1951 2,663,636 Middleton Dec. 22, 1953 (Gther references on following page) 9 i 10 UNITED STATES PATENTS 535,951 Belgium Mar. 15, 1955 2 4,044 Dalton D 29, 1953 533,514 Belgium 20, 1954 2,692,178 Granddadam Oct. 19, 1954 OTHER REFERENCES S535 "5 21' 3 5 Schaflert: Article on Xerography for Handbook of 2764693 Jacobo sgpt 261956 Photography, 21 proposed McGraw-Hiil publication in 2811465 Greig 1957 preparation as of November 1957, p. 21. 2825814 Nqlkul; 1958 Sugarman: The American Pressman, November 1955,

v pp. 33-38. 1

331 fi 10 Wainer: Phosphor-Type Photoconductive Coatings for Continuous Tone Electrostatic Electrophotography, 2875054 Gnggs etal 1959 Photographic Engineering, vol. 3, No. 8 (1952), pp.

FOREIGN PATENTS 1222. 201,301 Australia M 19 1 956 Young et aL: Electrofax, RCA Review, December 203,907 Australia Nov. 1, 1956 15 121 

1. A FLEXIBLE PHOTOCONDUCTIVE COPY SHEET HAVING A THIN, UNIFORM PHOTOCONDUCTIVE LAYER COMPRISING ZINC OXIDE PARTICLES FIRMLY HELD TO A SMOOTH CONTINUOUS FLEXIBLE ALUMINUM LAYER BY A SUBSTANTIALLY WATER-INSOLUBLE INSULATIVE ORGANIC RESINOUS BINDER HAVING A LOW DEGREE OF WETTABILITY TOWARD SAID ZINC OXIDE PARTICLES IN SUCH A MANNER THAT THERE IS A LOWERMOST STRATUM OF ZINC OXIDE PARTICLES DIRECTLY CONTACTING SAID ALUMINUM LAYER AND SUBSTANTIALLY ALL ZINC OXIDE PARTICLES IN SAID PHOTOCONDUCTIVE LAYER ARE IN DIRECT CONTACT WITH ADJACENT ZINC OXIDE PARTICLES, SAID ALUMIMUM LAYER OVERLAYING AND BEING ATTACHED TO AN ELECTRICAL INSULATING, FLEXIBLE BACKING SHEET, THE AMOUNT OF SAID ZINC OXIDE PARTICLES IN SAID PHOTOCONDUCTIVE LAYER BEING SUCH THAT AN ELECTRICALLY CONDUCTIVE PATH IS CREATED BETWEEN THE OUTER SURFACE OF SAID PHOTOCONDUCTIVE LAYER AND SAID ALUMINUM LAYER UPON IRRADIATION OF SAID PHOTOCONDUCTIVE LAYER, AND SAID ALUMINUM LAYER BEING OF SUCH CLEANNESS AND SAID ZINC OXIDE PARTICLES BEING OF SUCH PHOTOCONDUCTIVE LAYERS HAVE A CONBINDED ALIMINUM AND PHOTOCONDUCTIVE LAYERS HAVE A CONDUCTIVELY OF AT LEAST ABOUT 10-7 MHO/CM. ON EXPOSURE TO LIGHT AND A CONDUCTIVELY IN THE DARK NOT GREATER THAN ABOUT ONE-TWENTH OF THE CONDUCTIVELY ON EXPOSURE TO LIGHT. 